Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1999-09-22
2001-02-13
Scott, Jr., Leon (Department: 2881)
Coherent light generators
Particular active media
Semiconductor
C372S054000, C372S028000, C372S101000, C372S032000
Reexamination Certificate
active
06188708
ABSTRACT:
The invention relates to a single-frequency laser and amplifier system.
Nowadays, single-frequency lasers are required for many laser applications for communication tasks. While, in the communication via glass fibers, particularly laser diodes in the wavelength range of about 1.3 &mgr;m or 1.5 &mgr;m are customary for achieving a particularly high transmission or a particularly low dispersion in the glass fiber, for open-beam laser communication routes, for example, in space between satellites, lasers are also used at a shorter wavelength, for example, at 1.06 &mgr;m. Here, the specific requirements less concern the wavelength of the laser irradiation. The spectral narrow-band characteristic of the laser sources is much more important here, because, for the transmission of very high data rates (typically 650 Mbit/s and more) coherent transmission systems are used here.
Here, typical line width requirements are at <10 kHz or even less and a correspondingly low frequency jitter, which is generally not achieved by means of semiconductor lasers in the constant operation. In contrast, solid-state lasers are used here; specifically, for reasons of efficiency, preferably diode-pumped solid-state lasers. These have a coefficient, which is by approximately two orders lower, for changing the optical resonator length and thus the frequency of the laser irradiation with the temperature. In the case of semiconductor laser diodes, the coefficient amounts, for example, typically to 0.3 nm/° C.; at 830 nm, correspondingly 130 GHz/° C., in contrast to typically 3.5 GHz/° C. in the case of Nd;YAG lasers.
A transmitting laser for such an intersatellite transmission route typically requires an output power of >1W of continuous power while simultaneously meeting the line width specification. Further, the laser must be phase-modulated. However, modulators according to the state of the art operate only up to powers of a few 100 mW so that the transmitting laser power must first remain limited to this value and, after the modulation, must be amplified to the required nominal power.
According to the state of the art, diode-pumped solid-state amplifier arrangements are also used here which are constructed similarly to diode-pumped solid-state lasers but are operated below the lasing threshold. However, these amplifier arrangements require relatively high expenditures and are inefficient.
In the case of simple amplifier arrangements with only one or two passes, amplification factors of typically 1.5-2 are achieved here (W. Seelert et al.,
OSA Proc. on Advanced Solid-State Lasers
(Hilton Head, 1991), Vol. 10 (1991) 261) so that, for achieving an output power of 1 W from 100 MW oscillator power from the phase modulator, four or more amplifier stages are required.
In contrast, significantly higher amplifications of up to over 50 dB could be reached only by means of multipath amplifier arrangements (Kane et al.,
SPIE
, Vol. 2381, Pages 273; compare FIG.
2
). These arrangements require relatively high expenditures and, because of the complicated beam guidance, are subjected to high thermal fluctuations. The energy balance for such amplifier arrangements is also relatively poor (here 9.4 W electric input power+30 mW oscillator power resulted in 835 mW output power, electrical to optical amplifier efficiency of less than 9% W). In addition, these arrangements can also not be significantly miniaturized.
Semiconductor laser amplifiers are particularly simple which have a construction similar to that of semiconductor laser diodes of an epitaxial layer sequence of, for example, GaAs, GaAlAs, InGaAs or InGaAsP. In contrast to laser diodes, such semiconductor amplifiers have antireflex coatings on both end surfaces, so that the semiconductor element is operated far below the threshold power required for a laser operation as an oscillator. If laser irradiation is now coupled in on one side of the semiconductor element, this laser irradiation is amplified in the electrically pumped semiconductor material. Such arrangements have also been known for many years and are described, for example, in R. Waarts et al.,
Electron. Lett
. 26 (1990) 1926. For generating an irradiation of a high beaming quality, special structurings of the semiconductor amplifier are customary, such as broad-strip or trapezoid structures; compare J. N. Walpole et al.,
SPIE
, Vol. 1850, “Laser Diode Technology and Applications V (1993) 51.
Normally, semiconductor laser diodes of the same material are as a laser oscillator whose irradiation is to be amplified. Such oscillator amplifier structures (MOPA—Master Oscillator Power Amplifier) are preferably constructed on the same epitaxial substrate and are separated in their function by a corresponding structuring. Such components are described, for example, in R. Parke, CLEO 93
, Tech.Digest
, Contribution CTuI4 (1993) 108, and are offered commercially. However, these elements do not meet the specifications concerning line width and frequency jitter.
These MOPA structures are unsuitable for the above-mentioned tasks of coherent communication. The spatial beaming quality as well as particularly also the high line widths do not permit a coherent phase locking.
It is an object of the invention to indicate a simple, efficient and miniaturized laser and amplifier system which permits the generating of narrow-band laser irradiation in the watt range.
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“60mW 1.5 &mgr;m Single-Frequency Low-Noise Fiber Laser MOPA” G.A. Ball, C.E. Holton, G. Hull-Allen and W.W. Morey—Feb., 994 IEEE, pp. 192-194.
“High repetition rate femtosecond dye amplifier using a laser diode pumped neodymium: YAG laser” B. Zysset, MJ. LaGasse, J.G. Fujimoto, J.D.Kafka—320 Applied Physics Letters, Feb. 1989.
Koeniger Max
Pribil Klaus
Schmitt Nikolnus
Unger Peter
Contraves Space AG
Evenson, McKeown, Edwards & Lenahan P.L.L.C.
Jr. Leon Scott
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